Triple Play of Band Gap, Interband, and Plasmonic Excitations for Enhanced Catalytic Activity in Pd/HxMoO3 Nanoparticles in the Visible Region

Plasmonic photocatalysis has been limited by the high cost and scalability of plasmonic materials, such as Ag and Au. By focusing on earth-abundant photocatalyst/plasmonic materials (HxMoO3) and Pd as a catalyst, we addressed these challenges by developing a solventless mechanochemical synthesis of Pd/HxMoO3 and optimizing photocatalytic activities in the visible range. We investigated the effect of HxMoO3 band gap excitation (at 427 nm), Pd interband transitions (at 427 nm), and HxMoO3 localized surface plasmon resonance (LSPR) excitation (at 640 nm) over photocatalytic activities toward the hydrogen evolution and phenylacetylene hydrogenation as model reactions. Although both excitation wavelengths led to comparable photoenhancements, a 110% increase was achieved under dual excitation conditions (427 + 640 nm). This was assigned to a synergistic effect of optical excitations that optimized the generation of energetic electrons at the catalytic sites. These results are important for the development of visible-light photocatalysts based on earth-abundant components.


■ INTRODUCTION
Visible-light photocatalysis is a crucial technology to enable solar-driven chemistry and be the stepping stone for the green transition of our society. 1,2Photocatalysis can address important molecular transformations related to energy generation, synthesis of commodity chemicals, providing a clean environment, mitigating climate change, and enabling a circular economy. 3,4Nevertheless, photocatalytic performances in the visible region are hindered by the fact that most employed photocatalytic systems based on semiconductors require UV light for activation, limiting their efficiency for solar-driven chemistry. 5,6n this context, recent advances in plasmonic catalysis based on the localized surface plasmon resonance (LSPR) excitation in plasmonic nanoparticles (NPs) have enabled high photocatalytic enhancements in the visible range for a variety of transformations and catalyst designs. 7−14 Despite their potential, one of the bottlenecks is that most common plasmonic materials are composed of expensive metals such as Ag and Au NPs.Although alternative earth-abundant materials have been proposed, such as Al, Cu, metal nitrides, and metal oxides, 15−20 these systems may remain limited in terms of performance and large scalability of their synthesis.Therefore, the development of high-performing visible light photocatalysts that are based on abundant elements and that can be prepared by scalable and environmentally friendly processes is the piece of the puzzle needed to enable photocatalysis to reach its full potential.
In this paper, we address these challenges by focusing on H x MoO 3 as an earth-abundant photocatalytic and plasmonic semiconducting component (antenna) coupled with Pd NPs as the catalytic sites (reactor) in a model hybrid NP design.They are composed of Pd NPs supported onto defective H x MoO 3 , named Pd/H x MoO 3 .First, we prepared this material in one pot at room temperature and under solventless conditions by a mechanochemical approach followed by H-spillover.Then, we present a comprehensive investigation and optimization of photocatalytic properties in the visible region through the control of different optical excitation processes via distinct light illumination conditions.As H x MoO 3 supports band gap and LSPR excitation whereas Pd supports interband transitions (all in the visible range), this unique combination of excitation processes allows us to explore and understand how different light wavelengths impact the catalytic activity of Pd sites in the presence of H x MoO 3 .Here, we employed 427 nm to probe the effect of the H x MoO 3 band gap excitation and Pd interband transitions on catalytic activities, whereas 640 nm could be employed to focus on the impact of H x MoO 3 LSPR excitation.By investigating the hydrogen evolution reaction (HER) and phenylacetylene hydrogenation reaction as model transformations, we have found that both 427 and 640 nm excitation led to comparable enhancements in catalytic activity for both transformations.Intriguingly, we found that the catalytic activity was significantly boosted, achieving a 110% photoenhancement, when we employed dual excitation conditions (both 427 and 640 nm simultaneously).This could be explained by the synergistic effect of the H x MoO 3 band gap, H x MoO 3 LSPR, and Pd interband excitation, which collectively optimized the generation of energetic electrons at the catalytic sites (Pd) under light illumination.Therefore, these results provide valuable insights into the design and development of high-performance photocatalysts and the optimization of photocatalytic properties.

■ RESULTS AND DISCUSSION
The synthesis of Pd NPs supported on MoO 3 was performed via a mechanochemical process as described in Scheme 1.In this process, the Pd precursor was reduced by the NaBH 4 leading to the deposition of Pd NPs on MoO 3 while the MoO 3 structure was exfoliated (due to the mechanochemical forces). 21This leads to the formation of Pd/MoO 3 .During this process, H − species generated from NaBH 4 can also lead to some degree of H-intercalation onto the MoO 3 structure and oxygen removal. 21To endow Pd/MoO 3 with plasmonic properties (support LSPR in the visible region), hydrogen intercalation into the MoO 3 structure was performed via hydrogen spillover by bubbling H 2 gas into a suspension containing Pd/MoO 3 , leading to the formation of Pd/ H x MoO 3 .In this process, H 2 molecules undergo dissociative chemisorption on the Pd surface, forming adsorbed H atoms Scheme 1. Synthesis of Pd/H x MoO 3 Plasmonic Photocatalysts a a Scheme for the mechanochemical approach to produce Pd/MoO 3 followed by Pd-assisted hydrogen spillover to yield Pd/H x MoO 3 .−24 The intercalation of H-species leads to an increase in the presence of free charge carriers a with a high degree of delocalization, leading to LSPR in H x MoO 3 . 22,23As H x MoO 3 is also a semiconductor, in terms of photocatalysis applications, H x MoO 3 can support both band gap (semiconductor photocatalysis) and LSPR excitations (plasmonic catalysis) at different wavelengths. 25,26Moreover, it is established that Pd NPs can undergo interband excitations at wavelengths similar to the band gap excitation of H x MoO 3 . 27,28his makes this system, Pd/H x MoO 3 , ideal for investigating the effect of the following transitions to optimize catalytic activities under visible light excitation: (1) the combined effect of H x MoO 3 band gap excitation and Pd interband transitions; (2) the effect of the LSPR excitation of H x MoO 3 ; and (3) the triple play of H x MoO 3 band gap excitation, Pd interband transitions, and H x MoO 3 LSPR excitation.These are the goals that this paper aims to address.
The XRD patterns for the MoO 3 and Pd/MoO 3 materials are presented in Figure 1A.The diffraction peaks at 2θ: 12.7, 23.3, 25.7, 27.3, 33.8, 35.5, 38.9, 46.3, 49.2, and 58.8°were indexed to the (002), ( 110), (040), (021), ( 111), (041), (060), ( 210), (002), and (081) planes of orthorhombic MoO 3 (α-MoO 3 ) (JCPDS 005-0508). 29For Pd/MoO 3 , no peaks from Pd were observed because of the low Pd content in the sample (0.83 wt % as measured by AES).The minor reflections close to (110) and (040) detected for Pd/MoO 3 are assigned to the formation of the HxMO 3 structure, which is partially formed during the mechanochemical step. 21This is further illustrated by the XRD patterns obtained for Pd/ H x MO 3 after the H 2 bubbling step (Figure S1), which shows XRD peaks assigned to both MoO 3 and H x MO 3 phases.The crystallite sizes calculated by using the Scherrer equation corresponded to 43.1 and 35.8 nm for MoO 3 and Pd/MoO 3 , respectively.The decrease in crystallite size agrees with the mechanochemical synthesis that led to the exfoliation of the MoO 3 structure. 21Figure 1B shows the room temperature photoluminescence spectra of MoO 3 and Pd/MoO 3 obtained with a 320 nm excitation wavelength.−32 The lower energy bands located at 475 and 526 nm are related to electron−hole recombination between conduction band and defect sublevels created by surface defects or oxygen vacancies. 30,32For the Pd/MoO 3 spectrum, the signal intensities decreased, suggesting a strong interaction between the support MoO 3 and Pd NPs in which the presence of Pd improves the efficiency of electron−hole separation and suppresses the recombination of electron−hole pairs.
The Mo K-edge XANES spectra of as-synthesized MoO 3 and Pd/MoO 3 samples are shown in Figure S2, along with the Mo foil and commercial MoO 3 reference spectra.The commercial MoO 3 exhibited a discrete pre-edge peak at 20.005 keV followed by edge absorption at 20.02 keV, and both MoO 3 and Pd/MoO 3 exhibited similar features.The preedge feature is related to the quadrupole-allowed transitions from 1s to 4d levels, and the edge feature is related to the dipole-allowed transitions from 1s to 5p levels.Furthermore, the edge energy is indicative of an oxidation state, which is evidenced by the Mo foil spectra with a lower edge absorption (20.01 keV) compared to MoO 3 samples.The shift in absorption edges to higher energies indicates a higher oxidation state. 33,34o investigate the effect of H 2 bubbling and thus hydrogen spillover, the UV−vis absorption spectra of Pd/MoO 3 and Pd/ H x MoO 3 (before and after H 2 bubbling, respectively) are shown in Figure 1C.The as-synthesized Pd/MoO 3 showed a strong absorption band in the UV region due to band gap excitation typical of semiconductors, 35 with an absorption edge around 425 nm and discrete absorption in the visible and near-IR regions, which may be associated with a low degree of hydrogen doping in the MoO 3 structure during the synthesis with NaBH 4 .After the H 2 bubbling, the solution changed color from gray to dark blue accompanied by an increase in the LSPR peak from 450 to 900 nm, characteristic of plasmonic H x MoO 3 .The intercalation of H atoms into the van der Waals gap of MoO 3 induces intrinsic defects and injects electrons into the framework, changing the electronic structure, band gap, and conductivity. 22,29,36−39 These results showed that the H 2 bubbling step efficiently induces plasmonic properties into the MoO 3 support via the generation of H x MoO 3 due to hydrogen spillover, which is also supported by XRD data (Figure S1). 36,40,41In this process, H 2 interacts with the Pd surface, dissociates, and migrates into the MoO 3 lattice.This occurs through H atom adsorption onto oxygen, causing a charge transfer between the H 1s orbital and the O 2p orbital.In sequence, the O coordinates directly to Mo atoms transferring the extra charge, and the Mo atoms are slightly reduced. 36,40,41It is important to note that H x MoO 3 can be reversibly transformed to MoO 3 by gradual oxidation by O 2 in air under ambient conditions (in the period of 12−48 h). 15,42igure 2A,B shows transmission electron microscopy (TEM) images for the synthesized Pd/H x MoO 3 .It reveals that the H x MoO 3 was irregular in shape and that the supported Pd NPs were spherical, below 10 nm in diameter, and not uniformly distributed along the sample.This can probably be assigned to the incomplete reduction of the Pd precursors during the mechanochemical process (which was confirmed by the XPS data as discussed below).The HRTEM images (Figure 2C) showed apparent lattice spacings of 0.22 nm corresponding to the (111) plane of Pd structure (JCPDS no.46-104) and 0.35 nm assigned to the (040) plane of MoO 3 (ICSD 158256). 43,44The STEM-HAADF image (Figure 2D) revealed the presence of Pd NPs over the H x MoO 3 support, and STEM-EDX mapping showed that regions containing Mo and Pd were uniformly distributed throughout the sample (Figure 2E,F).
Figure 3 shows the Mo 3d and Pd 3d core-level X-ray photoelectron spectra of Pd/MoO 3 (before H 2 bubbling, Figure 3A,B) and Pd/H x MoO 3 (after H 2 bubbling, Figure 3C,D).The Mo 3d region in the spectra of Pd/MoO 3 (Figure 3A) can be readily fitted with a pair of symmetric components with peaks at 233.0 and 236.2 eV that correspond to the spin− orbit doublet characteristic of Mo (VI).The Pd 3d region (Figure 3B) is more complex, featuring four relative maxima that can be fitted with two pairs of Pd 3d spin−orbit doublets with peaks at 335.1 and 340.4 eV for one pair and at 337.9 and 343.2 eV for the second pair.These positions are characteristic of metallic Pd and Pd (IV) as in PdO 2 , respectively.Although the low spectral signal makes the quantification less reliable, we estimate from the relative peak areas that 25% of the Pd is in the metallic state.This result indicates that under our employed conditions, the mechanochemical process does not drive the complete reduction or conversion of the Pd precursor to metallic Pd.After H 2 bubbling (Pd/H x MoO 3 ), the Mo 3d spectrum (Figure 3C) displays a clear shoulder toward low binding energy values.This indicates the presence of a second component that can be fitted with an additional pair of spin− orbit doublets with positions at 231.4 and 234.5 eV.The position of this component is too high to associate it with Mo (IV), and it has been rather associated with the presence of Mo (V). 45This partial reduction of Mo (IV) to Mo (V) occurs because of the hydrogen spillover process due to H incorporation in this system.The Pd 3d spectrum for Pd/ H x MoO 3 (Figure 3D) can be well-fitted with a single pair of asymmetric components (at 335.1 and 340.4 eV) associated with metallic Pd, which indicates the complete reduction of Pd species to the metallic state under the H 2 bubbling step.The core-level spectra of the O 1s and C 1s are shown in Figure S3.Although no significant changes were detected in the core-level spectra of the O 1s after the H 2 bubbling step, an increase in the CO 2 chemisorption capacity of the sample was detected.This behavior has been also observed in MoO 3-x samples. 46,47he XANES spectrum for Pd/H x MoO 3 (Figure S2) shows a similar spectral profile relative to the Pd/MoO 3 and MoO 3 samples.This suggests that Mo 5+ sites are probably concentrated at the surface of the particles, being detected in XPS but not in XANES.
Our spectroscopic data (Figure 1C,D) show that the H x MoO 3 material presents both band gap and LSPR excitations, which are in different spectral regions.Whereas the wavelength for band gap excitation corresponds to 433 nm, the LSPR excitation band has its maximum intensities above 600 nm.In addition, Pd NPs present interband transitions in the range of less than 450 nm.Thus, in the next step, we were interested in (i) using different wavelengths to investigate and compare how the H x MoO 3 LSPR excitation (>600 nm) or a combination of H x MoO 3 band gap and Pd interband excitation (<450 nm) influences catalytic activities and (ii) using two wavelengths (dual excitation at both <450 and >600 nm) to investigate how the triple play of H x MoO 3 band gap, H x MoO 3 LSPR, and Pd interband excitation influences catalytic activities.To address this challenge, we started by employing a model catalytic transformation that is catalyzed by Pd only (and thus H x MoO 3 has negligible activity) and two different light irradiation wavelengths: 427 nm for the excitation of the H x MoO 3 band gap and Pd interband transitions and 640 nm for the excitation of the H x MoO 3 LSPR.It is noteworthy that we were interested in using only light in the visible region as the excitation source.
Figure 4A shows the LSVs for Pd/H x MoO 3 using dark (black traces) and light irradiation conditions at 427 or 640 nm (violet and red traces, respectively).The LSV curves indicate that the catalytic activity increases to a similar extent under both light illumination conditions.The overpotential at a current density of 10 mA cm −2 (η 10 ), which is a benchmark of HER performance, was measured.The lowest η 10 values (and thus higher activities) were obtained under light irradiation, corresponding to 202 and 210 mV under 427 and 640 nm irradiation, respectively.Under dark conditions, the overpotential was 227 mV.The decrease in overpotential as well as the increase in current density for Pd/H x MoO 3 at both wavelengths indicates that both H x MoO 3 LSPR excitation and H x MoO 3 band gap excitation and Pd interband transitions can contribute to enhancing HER activity, and these two effects led to a similar enhancement.The LSVs for pure H x MoO 3 shown in Figure S4A also revealed an increase in current density under light irradiation for both wavelengths, with a 2-fold enhancement compared to dark conditions.This agrees with the band gap and LSPR excitation in the H x MoO 3 .However, high overpotential values for pure H x MoO 3 were detected as expected due to its low catalytic activity for the HER. 48he Tafel plots presented in Figure 4B can provide important information about the HER reaction mechanism and rate-determining step.−51 The values suggest that the Volmer step (H + + e − + Pd ⇌ Pd−H ad ) is the rate- determining step. 52Chronoamperometric experiments under chopped light conditions for Pd/H x MoO 3 and the resultant photocurrent transients are reported in Figure 4C.The photocurrent generated under 427 and 640 nm irradiation promoted an average increase of 2.1 and 1.9 mA in current densities, respectively.A fast and reproducible current response to on−off illumination cycles was detected for both wavelengths, in agreement with the participation of photogenerated electrons from band gap excitation, interband, and LSPR excitation over the current enhancements.A similar effect was observed for H x MoO 3 as shown in Figure S4B.
These results show that both the H x MoO 3 LSPR excitation and the H x MoO 3 band gap excitation coupled with Pd interband transitions play important roles in enhancing the Pd catalytic activity toward the HER as illustrated in Figure 4D.Band gap excitation coupled with interband transitions (violet dashed line) leads to the formation of excited electrons that can be transferred to the Pd sites that, together with hot electrons from the interband transition, facilitate the HER process by reducing protons (H + ) to hydrogen (H 2 ).LSPR excitation in H x MoO 3 at 640 nm leads to the formation of hot electrons and holes.Here, hot electrons can be transferred to the Pd sites, also facilitating the HER by accelerating the reduction of H + species.Thus, the similar HER performances under both 427 and 640 nm illumination indicate a similar effect of the LSPR excitation in H x MoO 3 as compared to the H x MoO 3 band gap excitation coupled with Pd interband transitions.It is important to note that thermal effects (localized heating) under light illumination due to plasmonic excitation may also contribute to the enhanced HER activities, and the elucidation of thermal vs nonthermal effects in plasmonic catalysis is important. 53,54Nevertheless, the effects of excited or hot charge carriers relative to photothermal heating are difficult to disentangle under our experimental conditions, and it is plausible that both of these effects are contributing to the enhanced catalytic activities.Therefore, visible light excitation at 427 and 640 nm offers complementary mechanisms for improving the catalytic activity in the HER to a similar extent in Pd/H x MoO 3 .The similar enhancements observed at both wavelengths may indicate that better charge generation and utilization occur under plasmonic excitation (640 nm) under our conditions relative to Pd and H x MoO 3 interband electrons.However, this is difficult to evaluate precisely, as 427 nm does not correspond to the maximum absorption region for H x MoO 3 .It is noteworthy that the HER performances can be further improved in the future by optimizing the Pd loading onto the electrode and optimizing the synthesis.
DFT calculations on the electronic properties of Pd/ H x MoO 3 model NPs and the difference in Gibbs free energies (ΔG H* ) for the H* adsorption/desorption relative to Pd and H x MoO 3 model NPs were performed to obtain further insights into the detected HER activity (Figures S5−S7, see Supporting Information for further details). 55,56The simulation data agree with the boosting of HER activity in Pd/H x MoO 3 via the optimization of adsorption/desorption of H* species (under both dark and light conditions) and the facilitation of LSPRexcited charge carrier transfer (from H x MoO 3 ) to Pd NPs under light excitation. 56e then employed the hydrogenation of phenylacetylene as a model transformation, as depicted in Figure 5A, to investigate the effects of different light wavelengths as well as dual excitation at 427 and 640 nm over catalytic activity and reaction selectivity (our HER setup does not enable the use of dual excitation conditions).In this transformation, two products are possible: styrene (from semihydrogenation) and ethylbenzene (from full hydrogenation).Moreover, it is well established that Pd NPs have good activity toward alkyne hydrogenation reactions, whereas H x MoO 3 is not active.Therefore, as in the HER, the detected activities can be assigned to Pd. 57,58 Figure 5B (and Table S1) shows the phenylacetylene conversion % and the calculated photoenhancement percentage for the Pd/H x MoO 3 catalyst under different light irradiation wavelengths.GC and GC−MS analyses of the products are shown in Figures S8 and S9.The conversion % under dark conditions corresponded to 29%.Under light excitation conditions, an enhancement in the conversion % was observed for all the wavelengths and corresponded to 41, 39, 38, and 35% under 427, 640, 740, and 525 nm, respectively.This suggests relatively similar conversion percentages under light excitation as a function of the wavelengths, with the highest and lowest values corresponding to 427 and 525 nm, respectively.The highest photoenhancement was observed for 427 nm excitation, corresponding to 41%, which can be assigned to the band gap excitation of H x MoO 3 and interband transitions on Pd.Here, as discussed for the HER, visible excitation can lead to energetic electrons, which can contribute to accelerating the phenylacetylene hydrogenation reaction. 59,60According to our previous data on antenna−reactor plasmonic catalysts with Pd, the first hydrogenation step corresponds to the rate-determining step, and the excited electrons or hot electrons can contribute to accelerating this process. 59egarding the selectivity, all dark and light-irradiation conditions led to ≥95% selectivity for the formation of styrene (semihydrogenation reaction), as illustrated in Figure 5C, indicating that our catalyst was selective.The observed selectivity to styrene can be attributed to a two-step hydrogenation process.Initially, the phenylacetylene undergoes partial hydrogenation, leading to the formation of styrene.Subsequently, styrene undergoes further hydrogenation to produce ethylbenzene.As we maintained the conversion levels up to 60%, the reaction predominantly yielded styrene as the major product.This is supported by our experiments, where we expanded the scope of substrates for hydrogenation (Table S2).In instances where the substrate underwent a higher catalytic conversion (Table S2, entry 4), the fully hydrogenated product was detected.We calculated the turnover frequency (TOF) based on the Pd loading (0.83 wt %).The results are also presented in Figure 5C.TOF values ranged from 1239 in the dark to 1752 h −1 under 427 nm excitation.A comparison of the estimated TOF and other reported Pd-based catalysts is presented in Table S3.
After separately investigating the effect of distinct wavelengths over enhanced catalytic activities and selectivity under light excitation, which enabled us to separate the effect of H x MoO 3 LSPR from the H x MoO 3 band gap and Pd interband transition, we decided to unravel whether we could employ these three effects at the same time to achieve superior performance under dual excitation condition.In other words, we were interested in investigating the combined effect of the band gap and LSPR excitation from H x MoO 3 as well as the interband transitions from Pd over the catalytic activities and selectivity.This was achieved by employing both 427 and 640 nm as the light irradiation wavelengths to excite all of these processes.
Interestingly, by employing the dual excitation at 427 and 640 nm, a significant increase in the conversion % from 41% (under the best light irradiation conditions achieved at 427 nm) to 62% was detected.This corresponds to a 114% photoenhancement and a TOF of 2650 h −1 , with a selectivity toward styrene of 94%.This result shows that the dual excitation and thus the triple play of the H x MoO 3 band gap and LSPR excitation and Pd interband transitions lead to an enhancement in conversion %, which is superior to the sum of excitation at 427 or 640 nm only.This concept could be applied to a variety of substrates as shown in Table S2, suggesting the versatility of this approach.Figure 6 summarizes the roles of the H x MoO 3 band gap and LSPR excitation and Pd interband transitions over the enhanced catalytic activities under 427, 640, and dual 427 + 640 nm excitation conditions.When 427 nm excitation is employed (Figure 6A,B), light absorption occurs via band gap excitation on H x MoO 3 (Figure 6A) and interband transitions in Pd (Figure 6B).This leads to the formation of energetic electrons from Pd and from H x MoO 3 , which can be transferred to Pd.Both of these effects contribute to accelerating the phenylacetylene hydrogenation reaction by activating adsorbed molecules (phenylacetylene or H-species) via electronic or vibrational excitation, accelerating the first hydrogenation step (rate-determining step).When 640 nm excitation is employed, as shown in Figure 6C, LSPR excitation in H x MoO 3 generates hot electrons that are then transferred to the Pd NPs, also leading to enhanced activities.Finally, when both 427 and 640 nm are employed as light irradiation wavelengths (Figure 6D), all these processes, i.e., band gap and LSPR excitation from H x MoO 3 and interband transitions from Pd, lead to the formation and transfer of energetic electrons to the Pd sites, leading to a superior enhancement in catalytic activity relative to the separate effect of each of these light wavelengths.These results demonstrate that the control and optimization over the light irradiation and optical excitation processes using dual excitation conditions can lead to the maximization of catalytic activities in the visible range, opening new avenues for the optimization of photocatalytic and plasmonic properties.
Finally, we performed DFT calculations to gain insights into the observed activity toward the hydrogenation of phenylacetylene (Figures S10−S15).Figure S10 shows that phenylacetylene adsorbs at the Pd and Pd/H x MoO 3 surfaces in a flat configuration relative to Pd due to the interaction of the aromatic ring with the surface. 59The PDOS for phenylacetylene before and after adsorption is shown in Figure S11A and reveals the strong interaction at the surface with the downshifted 2p orbitals.The calculated charge density differences (Figure S11B) agree with this observation and suggest a stronger local charge redistribution relative to the pure Pd (111) model (Figure S12).To quantitatively estimate the interaction between Pd sites and adsorbed phenylacetylene, the projected crystal orbital Hamilton population (pCOHP) of the Pd−C bond (Pd in the surface site and terminal C in adsorbed phenylacetylene, Figure S13) was examined.The integral COHP value (ICOHP) could be calculated by integrating the partial COHP below the E F , suggesting the number of bonded electrons between the selected Pd and C atoms and the corresponding bonding strength. 56It was clear that the ICOHP of Pd−C in Pd/MoO 3-x was larger than that of Pd (111) and indicated stronger phenylacetylene adsorption (Figure S11C).
The calculated hydrogenation reaction pathway and changes in energy for the formation of styrene and ethylbenzene are shown in Figures S11D and S14.The calculated maximum energy barriers for the hydrogenation of phenylacetylene to styrene on Pd/H x MoO 3 and Pd (111) models were 0.46 and 0.68 eV, respectively.To achieve selective hydrogenation toward styrene, the desorption of adsorbed styrene (PhCHCH 2 *) is key.As shown in Figure S11D, the styrene desorption energy on Pd/H x MoO 3 is further reduced relative to that on Pd (111) (0.70 and 1.14 eV, respectively), indicating that the presence of the H x MoO 3 further contributes to driving the selectivity for semihydrogenation in this system.The simulations of the charge density difference results (Figure S15) also indicate that the adsorption of styrene over Pd (111) is stronger than that on Pd/H x MoO 3 .Finally, as described for the HER, it is plausible that thermal effects may also contribute to the enhanced reaction rates under light excitation conditions.

■ CONCLUSIONS
We report herein the investigation and optimization of photocatalytic properties in the visible region by controlling different optical excitation processes via distinct light illumination conditions.To this end, we employed Pd NPs supported onto H x MoO 3 (Pd/H x MoO 3 ) as a model catalyst, as this system supports band gap and LSPR excitation (from H x MoO 3 ) as well as interband transitions (from Pd).The Pd/ H x MoO 3 was obtained by hydrogen spillover of Pd/MoO 3 , which was prepared by a one-pot mechanochemical synthesis under solventless conditions.Whereas the H x MoO 3 band gap excitation and Pd interband transitions can be excited at 427 nm, the H x MoO 3 LSPR excitation is maximized at >600 nm.Therefore, by employing 427 nm as the excitation wavelength, we could probe the effect of H x MoO 3 band gap excitation and Pd interband transitions over catalytic activities, and by employing 640 nm, we could probe the effect of the LSPR excitation.We employed the HER and the phenylacetylene hydrogenation as model transformations to study how these different excitation conditions affect the catalytic activities from Pd (catalytic sites).Our data show that either 427 or 640 nm excitation led to similar enhancements in catalytic activity for both transformations and that the hydrogenation of phenylacetylene was selective for the formation of styrene.Interestingly, we have found that the catalytic activity toward the phenylacetylene hydrogenation was maximized when we employed dual excitation conditions, i.e., employing both 427 and 640 nm as the light irradiation conditions.In this case, an increase in the photoenhancement to 110% was achieved relative to 41% (under 427 nm irradiation) and 39% (under 640 nm irradiation).Moreover, the catalyst was still selective toward the formation of styrene (94%).This increase in catalytic activity under dual light excitation conditions was assigned to the triple play of the H x MoO 3 band gap, H x MoO 3 LSPR, and Pd interband excitation.These processes optimize and maximize the formation of energetic electrons at the catalytic sites (Pd) under light illumination conditions, allowing for the superior acceleration of the phenylacetylene hydrogenation.We believe that the results reported herein provide important insights for the future design and development of higher performing photocatalysts, in which the ability to control optical excitations can provide an important avenue to maximize catalytic activities.Coupled with green and solventless catalyst synthesis methods that enable large-scale production, these findings can bring solar-driven chemistry for a sustainable planet one step closer to reality.
Transmission electron microscopy (TEM) measurements were conducted on a JEOL JEM-2200FS microscope operating with an accelerating voltage of 200 kV.The samples were prepared by drop casting an aqueous suspension onto a carbon film on a copper TEM grid (400 square mesh) and drying under ambient conditions.Powder X-ray diffraction (PXRD) analysis was performed on a Bruker D8 Advance diffractometer with a Cu Kα radiation source (λ = 1.5406Å) and a Ni filter.The diffraction patterns were collected in the 2θ range of 10−70°, with a 0.02°step width and 1 s/step count time.The calculation of the crystallite size was performed by the Scherrer equation: where D is the average crystallite size, K is the shape factor (adopted to 0.94 for roundish particles), B is the fwhm in radians of a peak at the angle theta, and lambda is the wavelength (0.154 nm for Cu Kα).
Atomic emission spectroscopy (AES) analysis was performed on an Agilent Technologies 4100 MP spectrometer to determine the Pd content.Pd standard solutions of 1, 2, 5, 7, and 10 ppm were prepared using K 2 PdCl 4 salt dissolved in 0.5 M HCl solution.The catalyst was dissolved in aqua regia followed by the evaporation of the solvent on a hot plate at 50 °C.Then the catalyst was redispersed in 0.5 M HCl solution, resulting in a Pd concentration of 5 ppm.UV−vis analysis was conducted in a Shimadzu UV-2600 PC spectrophotometer using quartz cuvettes with an optical path of 1 cm.The sample was dispersed in isopropanol and purged for 5 min with H 2 before the measurement.The photoluminescence (PL) spectra of the catalysts (powder samples) were recorded at room temperature using a Horiba FluoroMax-4 Spectrofluorometer equipped with a 150 W xenon arc lamp as the excitation source and a Front Face sample holder.All the spectra were recorded at room temperature.
X-ray photoelectron spectra were recorded with a lab-based spectrometer (SPECS GmbH, Berlin) using a monochromated Al Kα source (hν = 1486.6eV) operated at 50W as the excitation source.In the spectrometer, the X-ray was focused with a μ-FOCUS 600 monochromator onto a 300 μm spot on the sample, and the data were recorded with a PHOIBOS 150 NAP 1D-DLD analyzer in fixed analyzer transmission (FAT) mode.The pass energy was set to 40 eV for the survey scans and 20 eV for the high-resolution regions.The binding energy scale was calibrated using Au 4f 7/2 (84.01 eV) and Ag 3d 5/2 (368.20 eV).Charge compensation was required for data collection.Recorded spectra were additionally calibrated against the C 1s internal reference.Data interpretation was performed with Casa XPS.Shirley or two-point linear background was used depending on the spectrum shape.
The X-ray absorption near edge spectra (XANES) were measured using the Rowland circle spectrometer Hel-XAS 61 of the Helsinki Center for X-ray Spectroscopy equipped with a spherically bent Si (12,12,0) crystal analyzer (bending radius 0.5 m).The Bremsstrahlung from a water-cooled Ag-anode X-ray tube was monochromated and focused by the crystal onto a Si drift diode detector.Scanning the spectrometer Bragg angle across the Mo K absorption edge energy yielded the intensity I(E) with and I_0(E) without the sample, from which the dimensionless XANES spectrum \mu(E) × d was obtained, d being the sample thickness.A linear pre-edge background was subtracted from the spectra, and the spectra were normalized to equal area.
Synthesis of Pd/MoO 3 and Pd/H x MoO 3 .The synthesis of supported Pd nanoparticles in MoO 3 was prepared by partial reduction of the Pd precursor and commercial MoO 3 using sodium borohydride (NaBH 4 ) in a ball-milling device following a previous method. 21The mechanochemical synthesis was performed on a vertical vibratory ball mill (Pulverisette 23, Fritsch) at 50 Hz using a PMMA milling jar (10 mL) with a single zirconia milling ball (diameter of 10 mm; 3.14 g).In a typical procedure, commercial MoO 3 (974 mg) and K 2 PdCl 4 (10 mg) were milled for 10 min to achieve a good dispersion; then NaBH 4 (26 mg) was added to the jar, and the mixture was milled for 1 h.The resultant Pd/MoO 3 powder was washed with water and ethanol by centrifugation, resuspension, and removal of the supernatant several times.The final product was dried at 80 °C for 12 h.For comparison, commercial MoO 3 (974 mg) and NaBH 4 (26 mg) were ball milled for 1 h without the Pd precursor, and the resultant MoO 3 powder was washed and dried following the same procedure described above.Pd/H x MoO 3 NPs were prepared by bubbling a suspension of the obtained Pd/MoO 3 (5g/L) with H 2 gas for 5 min at 1 atm.
Electrocatalytic Tests: Hydrogen Evolution Reaction (HER).A water-jacketed three-electrode glass cell was used during the electrochemical experiments, and the temperature was kept at 25 °C by using a water bath and a refrigerated circulator (Julabo F12-MA).A glassy carbon electrode (6 mm diameter, geometric area of 0.2827 cm 2 ) was used as a working electrode, a graphite rod was used as a counter electrode, and a reversible hydrogen electrode (RHE) was used as a reference electrode.The GC electrode was cleaned by polishing with alumina slurry (0.05 μm) and an ultrasonic bath in ultrapure water and acetone for 5 min each.Then 5 mg of the catalyst and 1 mg of carbon Vulcan XC-72R were dispersed in 1 mL of isopropanol/water (3:1) solution (containing 0.1 wt % Nafion) to form a homogeneous ink, and 30 μL of the dispersion was dropcast on the GC electrode.The total catalyst loading was 500 μg/cm 2 , and the Pd loading for the Pd/MoO 3 catalyst was ≈4 μg/cm 2 .Linear sweep voltammetry (LSV) measurements were performed at a scan rate of 5 mV s −1 , and chronoamperometry (CA) measurements were recorded at −0.2 and −0.5 V for Pd/MoO 3 and MoO 3 , respectively, both in Ar-saturated 0.5 M H 2 SO 4 solution.For photoelectrocatalytic experiments, the cell was irradiated by Kessil PR 160L LED with 640 or 427 nm irradiation, resulting in a total irradiance of 120 or 142 mW cm −2 , respectively.The electrochemical measurements were carried out using an Autolab PGSTAT 128N equipped with a Scan 250 modulus potentiostat.Before the experiments, the solution was purged with Argon 2.2, and during the data collection, this gas was kept in the cell headspace.
Catalytic Tests: Hydrogenation of Phenylacetylene.The hydrogenation of phenylacetylene was performed using a 100 mL Schlenk flask.Initially, 1.5 mg of the Pd/MoO 3 catalyst and 0.5 mmol of phenylacetylene were suspended in isopropanol, reaching a total volume of 10 mL, and the flask was kept for 10 min in an ultrasound bath to obtain a homogeneous dispersion.The mixture was purged with hydrogen for 5 min, and a balloon filled with H 2 at a pressure of 1 bar was used to seal the flask.Two flasks were prepared at the same time to run the reactions under dark and light conditions separately.For the dark (light off) experiment, the reactor was covered with aluminum foil.For the irradiation experiment (light on), four Kessil PR 160L LEDs with 427, 525, 640, or 740 nm irradiance (Kessil PR 160) were positioned 6 cm away from the reactor, resulting in a total irradiance of 14.34 mW/cm 2 according to the experimental setup reported elsewhere. 62A cooling fan was also placed above the reactor to avoid overheating.The experiments with two different wavelengths were performed using two 427 and two 740 nm LEDs in a similar configuration and total irradiance as described above.For all experiments, the reactors were kept at 40 °C in a silicon bath, the solutions were stirred using a Teflon-coated magnetic stirrer, and the reaction was carried out for 1 h.
Gas chromatography (GC) was performed at the end of the reaction to analyze the products.Two aliquots of 1.5 mL were collected from each reactor, centrifugated for 10 min at 13,000 rpm to remove the catalyst, and filtered using syringe filters (0.22 μm filter diameter).Afterward, the liquid sample was injected in a NEXIS GC-2030 gas chromatograph coupled to a flame ionization detector and a Crossbond SH-Rxi-5 ms capillary column (30 m × 0.25 mm × 0.25 μm), allowing for the separation and quantification of the reaction products.The heating program used during the analysis was as follows: 70 °C for 1 min, 6 °C min −1 until 85 °C, and then 100 °C min −1 until 300 °C.For the t = 0 samples, a solution containing 0.5 mmol of phenylacetylene in isopropanol was injected before each catalytic test using the same procedure described above.
The phenylacetylene conversion (X PA %) was calculated using eq 1: where A PAi and A PAt are the chromatographic areas of phenylacetylene in the samples at time = 0 and t, respectively.The product selectivity was calculated using eq 2: where A pi and ∑A j are the chromatographic area of the product p i and the sum of the chromatographic area of all products, respectively.The turnover frequency (TOF) was estimated by dividing the number of moles of reactant converted per number of active sites per time.
Computational Methods.DFT simulations were run with the first-principles simulation DMol3 module of Materials Studio. 63The generalized gradient approximation of Perdew−Burke−Ernzerhof exchange-correlation functionals was adopted for all the calculations. 64Core treatment was adopted as all electron to conduct the metal relativistic effect, and the double numerical plus polarization function basis set was used.A smearing of 0.01 Ha to the orbital occupation and a 1 × 10 −5 Ha convergence criterion for selfconsistent-field (SCF) calculations were applied.The geometry optimization convergence tolerance for energy change, max force, and max displacement were 1 × 10 −5 Ha, 0.004 Ha/Å, and 0.005 Å, respectively.The vacuum spacing in the direction along the Z axis with respect to the surface was 20 Å between neighboring slab images, which is sufficient to eliminate the interactions between the slabs.Activation barriers were obtained based on the linear synchronous and quadratic synchronous transit LST/QST. 65he free energy (ΔG) calculations of each elementary step were based on the standard hydrogen electrode model, 66 and the reaction free energy change can be obtained with the equation below: where ΔE is the total energy difference before and after intermediate adsorption and ΔE ZPE and ΔS are, respectively, the differences in zero-point energy and entropy, respectively.For the HER, the hydrogen-adsorption Gibbs free energy (ΔG H* ) was calculated according to the equation below: where ΔE H is the hydrogen absorption energy, ΔE ZPE is the correction of the zero-point energy, T is the temperature setting at 298.15 K, and ΔS H is the entropy difference between the absorbed hydrogen atom and free H 2 molecule, and ΔE ZPE -TΔS H is approximately 0.24 eV.

■ ASSOCIATED CONTENT
* sı Supporting Information

Figure 1 .
Figure 1.Characterization of Pd/MoO 3 and Pd/H x MoO 3 samples.(A) Powder XRD patterns and (B) photoluminescence spectra for the Pd/ MoO 3 and MoO 3 materials.(C) UV−vis extinction spectra and (D) corresponding Tauc plots for Pd/MoO 3 and Pd/H x MoO 3 (before and after H 2 bubbling) dispersed in isopropanol.MoO 3 , Pd/MoO 3 , and Pd/H x MoO 3 are denoted by red, black, and blue traces, respectively.

Figure 2 .
Figure 2. Electron microscopy characterization for Pd/H x MoO 3 NPs.(A, B) TEM, (C) HRTEM, and (D) STEM-HAADF images for the Pd/ H x MoO 3 NPs.(E, F) STEM-EDX mapping for (E) Mo and (F) Pd of the region shown in panel D. The distributions of Mo and Pd are shown in blue and red, respectively.

Figure 3 .
Figure 3. XPS characterization for Pd/MoO 3 and Pd/H x MoO 3 .High-resolution photoelectron spectra of (A and B) Pd/MoO 3 and (C and D) Pd/ H x MoO 3 in the Mo 3d (A, C) and Pd 3d (B, D) regions.

Figure 4 .
Figure 4. Electrocatalytic performance toward the HER under different light excitation conditions.(A) LSV curves for Pd/H x MoO 3 performed in the dark and under 427 and 640 nm LED irradiation conditions recorded at 5 mV s −1 , (B) Tafel plots calculated from LSV curves, (C) chronoamperometries recorded at −0.2 V under chopped illumination in Ar-saturated H 2 SO 4 0.5 M solution, and (D) schematics illustrating the photoelectrochemical HER using Pd/H x MoO 3 as photocathode and graphite electrode as counter electrode.

Figure 5 .
Figure 5. Photocatalytic hydrogenation of phenylacetylene over Pd/H x MoO 3 catalyst in the dark and under 427, 525, 640, and 740 nm LED irradiation conditions.(A) Reaction scheme, (B) total conversion and photoenhancement, and (C) selectivity for the hydrogenation of phenylacetylene to styrene and turnover frequency (TOF).The reactions were conducted under an H 2 atmosphere (1 bar) with 10 mL of isopropanol, 0.5 mmol of the substrate, and 1.5 mg of catalyst.The reaction time was 1 h.

Figure 6 .
Figure 6.Proposed scheme for the energy diagram and optical excitations for Pd/H x MoO 3 .(A) H x MoO 3 band gap excitation at 427 nm; (B) Pd interband transitions at 427 nm; (C) H x MoO 3 LSPR excitation (640 nm); and (D) triple play of the H x MoO 3 band gap and LSPR excitation and Pd interband transitions under dual illumination.